With the recent increase of consciousness of the importance of environmental conservation and environmental purification, studies for replacing conventional every-day materials with those made of biodegradable materials have been conducted on a world-wide scale.
Among biodegradable materials which have been proposed to date, starch-type naturally-occurring substances have poor water resistance; thus, their use is somewhat confined from a practical perspective. Imparting water resistance to such biodegradable materials would broaden their application and furnish solutions to various problems confronting the environment, particularly in terms of waste disposal.
Articles with poor water resistance may be coated to waterproof them. In taking this approach, it is required to establish a technique for producing with ease a biodegradable thin film which exhibits sufficient water resistance so as not to absorb water, not to become swollen with water and not to be weakened even when contacted with water for a long time. In particular, films made of a naturally-occurring substance which are completely degradable have been attracting considerable attention.
However, currently available biodegradable films made of naturally-occurring substances are strictly limited in their use due to their poor water resistance. Therefore, such materials are virtually impractical for use as containers for foods having a significant water content.
ZEIN, a catalogue published by Freeman Industries, Inc. discusses zein, a main protein of corn (Zea mays), as a coating material. Although zein itself and films of zein prepared in a known manner do not dissolve in water, zein and films thereof are known to absorb water, and thus become weak. Therefore, conventional zein films are not water-resistant and find little, if any, practical utility.